- 1 1/2 pound London broil
- 1/2 cup olive oil, plus more for brushing steak
- Salt and pepper
- 1/4 cup balsamic vinegar
- 4 portobello mushroom caps, cut in slices
- 1 Vidalia onion, cut in rings
- 1/2 cup mayonnaise
- 1 tablespoon grated horseradish or grainy mustard
- 1/2 cup chopped fresh parsley
- 1/2 lemon, juiced
- 4 pieces ciabatta bread, split horizontally
- 1/4 pound Fontina, sliced
- 1/3 cup (packed) coarsely chopped arugula leaves
Preheat oven broiler for 5 minutes.
Brush both sides of London broil with olive oil and season with salt and pepper. Place the steak on a broiler pan and broil about 4-inches from the heat source, about 5 minutes per side. Lay the portobellos and onions in a single layer on a sheet pan lined with aluminum foil. Drizzle with oil and vinegar then put in the oven and roast for 10 minutes. Remove the steak to a cutting board. Let rest for 5 minutes then slice the steak thinly against the grain.
In a small bowl, combine mayonnaise, horseradish, parsley, and lemon juice. Spread mayonnaise mixture on both sides of the split ciabatta. Lay several slices of steak on the bread and mound with a bit of mushrooms and onions. Cover with a couple of slices of cheese. Garnish with arugula, and serve.